This project is investigating the role of sensory processes and oculomotor tonus in the control of visual accommodation and binocular vergence. The efficiency of these responses varies widely across viewing conditions and individuals. In many situations, accommodation and vergence adjustments are biased toward the individual's characteristic resting (tonus) states, resulting in anomalous refractive errors and fixation disparity which can seriously interfere with normal perception and performance. In addition, recent evidence indicates that near work can modify the resting tonus and response characteristics of accommodation and vergence, and suggests that these changes may contribute to asthenopia and the development of myopia. Unobtrusive techniques, such as laser optometry and photorefraction, are being used to study (1) specific variables which modulate the resting tonus of accommodation and vergence; (2) the role of contrast sensitivity and binocular sensory integration in controlling accommodation; and (3) the nature of developmental changes in the sensory control of accommodation. The results of this research should enhance our understanding of the central mechanisms participating in oculomotor control and their influence on visual performance. They may also provide new clues to the role of accommodation and vergence in disorders such as strabismus, amblyopia, myopia, and asthenopia.